Tendinopathies and Pain Sensitisation: A Meta-Analysis with Meta-Regression

The presence of pain sensitisation has been documented and reported as being a possible cause of treatment failure and pain chronicity in several musculoskeletal conditions, such as tendinopathies. The aim of the present study is to analyse existing evidence on pain sensitisation in tendinopathies comparing the local and distant pain thresholds of healthy and affected subjects with distinct analysis for different tendinopathies. PubMed, Cochrane Central Register, Scopus, and Web Of Science were systematically searched after registration on PROSPERO (CRD42020164124). Level I to level IV studies evaluating the presence of pain sensitisation in patients with symptomatic tendinopathies, documented through a validated method, were included. A meta-analysis was performed to compare local, contralateral, and distant pain thresholds between patients and healthy controls with sub-analyses for different tendinopathies. Meta-regressions were conducted to evaluate the influence of age, activity level, and duration of symptoms on results. Thirty-four studies out of 2868 were included. The overall meta-analysis of local pressure pain thresholds (PPT) documented an increased sensitivity in affected subjects (p < 0.001). The analyses on contralateral PPTs (p < 0.001) and distant PPTs (p = 0.009) documented increased sensitivity in the affected group. The results of the sub-analyses on different tendinopathies were conflicting, except for those on lateral epicondylalgia. Patients’ activity level (p = 0.02) and age (p = 0.05) significantly influenced local PPT results. Tendinopathies are characterized by pain sensitisation, but, while features of both central and peripheral sensitisation can be constantly detected in lateral epicondylalgia, results on other tendinopathies were more conflicting. Patients’ characteristics are possible confounders that should be taken into account when addressing pain sensitisation.


Introduction
Tendinopathies are common injuries, frequently sport-related and affecting a young and active population [1][2][3][4]. The characteristics of the affected patients imply a relevant economic burden for society [5], which is also increased by the difficulties in properly addressing the disease with the available treatments, often leading to symptoms' chronicity. As well as tenderness to palpation and impaired function, tendon pain with loading is the main symptom of tendinopathies but their origins are still not well understood, thus undermining the treatment possibilities [6,7].

Data Source
After the registration of the protocol on PROSPERO (CRD42020164124), PubMed (MEDLINE), the Cochrane Central Register of Controlled Trials (CENTRAL), Scopus, Web of Science, and gray literature were systematically searched on 1 March 2022.
The following string was used: (tendon OR tendinopathy OR tendinopathies OR epicondylalgia OR tennis elbow OR jumpers knee OR patellar tendon OR Achilles OR rotator cuff OR shoulder impingement) AND (pain) AND (sensitization OR hyperalgesia OR threshold OR hypersensitivity OR algometry OR allodynia OR quantitative sensory testing OR neuropathic). No Institutional Review Board permission was needed to retrieve data.

Study Selection
The study selection process was performed between 2 March and 14 March 2022. Firstly, duplicates were removed, and then all titles and abstracts were checked to retrieve all eligible articles. The full-text article was read if not enough information on eligibility could be obtained from the abstract. Inclusion criteria were: Level I to level IV studies; studies evaluating the presence of pain sensitisation through a validated method (questionnaires or QST) or comparing QST between healthy and affected subjects; studies including patients with symptomatic chronic (pain duration > 3 months) tendinopathies. No time or language limitations were set. The Preferred Reporting Items for Systematic Reviews and Metaanalysis (PRISMA) guidelines were used [16]. Two authors (DP, AM) independently performed the article selection process, with any disagreement on study eligibility solved by a third author (GF).

Data Extraction and Study Outcomes
The data extraction process was performed between 15 March and 10 April 2022. Information on methodology from all eligible studies included level of evidence, publication year, study design, technique of pain sensitisation assessment, type of tendinopathy evaluated and diagnostic technique, origin of patients, number of patients included, and follow-up length. Information from all eligible trials on the characteristics of the study population included: inclusion/exclusion criteria, sex, age, body mass index, activity level, comorbidities, PROMs, duration of symptoms, functional scores, presence of pain sensitisation (number of patients or prevalence), results of pain sensitisation assessment (questionnaires, QST), and the influence of pain sensitisation on the clinical outcome. Two review authors extracted the trials' information independently and in duplicate using a standardized extraction form. The primary outcome was the difference in local pressure pain threshold (PPT) between patients with tendinopathies and healthy subjects. Contralateral and distant PPT, local heat pain threshold (HPT), and local cold pain threshold (CPT) were also evaluated. Separate analyses for different tendinopathies were conducted, with a sub-analysis based on the level of activity of the included patients. The influence of age and activity level of the patients on the results of the meta-analyses was evaluated through a meta-regression since they were identified in the literature as possible predictors of pain sensitisation [15]. An evaluation of the differences in terms of conditioned pain modulation and temporal summation was not possible due to the lack of data.

Quality Assessment
The quality of the included studies was assessed by two separate authors (DP, AM) using the Newcastle-Ottawa scale for case-control studies [17]. This uses a star-rating system to judge the study quality based on case selection (case definition, representativeness of cases, controls' definition, and controls' selection), comparability of cases and controls (in terms of age and other variables), and exposure (ascertainment of exposure, same method of ascertainment between cases and controls, and non-response rate). In the case of disagreement between the two authors, the studies were discussed and a consensus was reached.

Statistical Analysis
Continuous data were expressed as means and standard deviations and compared as standardized mean differences, whereas binary data were expressed as frequencies and compared as risk ratios. A meta-analysis was performed to compare local, contralateral, and distant PPTs, local HPTs, and local CPTs between patients with painful tendinopathies and healthy controls. Three separate linear meta-regressions were conducted to evaluate the influence of patient age, activity level, and duration of symptoms on the reported results. A multiple meta-regression was not feasible due to the low number of included studies [18]. Separate analyses for different tendinopathies were conducted. The random effect model with Knapp-Hartung-Sidik-Jonkman adjustment was used and results were expressed as standardized mean differences (SMD). The statistical analysis was performed with meta (v4.9-7, Schwarzer G, 2007) and metafor (v2.1-0, Viechtbauer, W., 2010) packages in RStudio (v1.2.5019; 250 Northern Ave., Boston, MA, USA, 02210).

Characteristics of the Included Studies and Patients
This meta-analysis included 34 studies out of 2868 retrieved records ( Figure 1) [11,12,. These 34 studies addressed 9 different tendinopathies: 11 were on lateral epicondylalgia, 5 were on Achilles tendinopathy, 4 were on shoulder impingement syndrome, 3 were on great trochanter pain syndrome, 3 were on patellar tendinopathy, 2 were on rotator cuff tears, 2 were on plantar heel pain, 2 were on all lower limb tendinopathies (greater trochanteric pain syndrome, quadriceps tendinopathy, patellar tendinopathy, Achilles tendinopathy, and plantar heel pain), 1 was on ilio-tibial band syndrome, and 1 included both patients with patellar and Achilles tendinopathy. Twentyeight studies evaluated the differences between affected and healthy subjects in terms of pain thresholds, whereas 6 studies (2 on rotator cuff tears, 1 on greater trochanteric pain syndrome, 1 on Achilles tendinopathy, and 2 on lower limb tendinopathies) reported the prevalence of features of pain sensitisation in a cohort of patients using validated ques-Biomedicines 2022, 10, 1749 4 of 23 tionnaires (i.e., painDETECT, Dolour Neuropathic 4, Central Sensitization Index) or QST protocols (conditioned pain modulation). syndrome, and 1 included both patients with patellar and Achilles tendinopathy. Twentyeight studies evaluated the differences between affected and healthy subjects in terms of pain thresholds, whereas 6 studies (2 on rotator cuff tears, 1 on greater trochanteric pain syndrome, 1 on Achilles tendinopathy, and 2 on lower limb tendinopathies) reported the prevalence of features of pain sensitisation in a cohort of patients using validated questionnaires (i.e., painDETECT, Dolour Neuropathic 4, Central Sensitization Index) or QST protocols (conditioned pain modulation). A total of 1831 patients were included: 406 with lateral epicondylalgia, 338 with Achilles tendinopathy, 290 with great trochanter pain syndrome, 287 with plantar heel pain, 211 with rotator cuff tears, 188 with patellar tendinopathy, 87 with shoulder impingement syndrome, 9 with ilio-tibial band syndrome, and 4 with quadriceps tendinopathy. In the reported studies the mean age ranged from 21.9 to 65.7, the mean BMI ranged from 22.2 to 33.7, and the mean pain duration ranged from 3 months to 10.6 years. Details on the characteristics of the included studies and patients are reported in Table 1. A total of 1831 patients were included: 406 with lateral epicondylalgia, 338 with Achilles tendinopathy, 290 with great trochanter pain syndrome, 287 with plantar heel pain, 211 with rotator cuff tears, 188 with patellar tendinopathy, 87 with shoulder impingement syndrome, 9 with ilio-tibial band syndrome, and 4 with quadriceps tendinopathy. In the reported studies the mean age ranged from 21.9 to 65.7, the mean BMI ranged from 22.2 to 33.7, and the mean pain duration ranged from 3 months to 10.6 years. Details on the characteristics of the included studies and patients are reported in Table 1. Active participation in regular physical activity > 3 d/w, pain at the insertion or mid-portion of the AT > 3 m, pain with palpation to the involved AT and its insertion, pain with AT loading activities AT tears, AT surgery, chronic pain, or inflammatory condition. Achilles region symptoms with screening tests for lumbar problems. Use of pain medications, SSRI, neurological condition, other orthopedic injury to the spine or lower extremities < 1 y, loss of sensation to the lower legs.
No musculoskeletal pain conditions < 1 y. Active participation in regular running (>5 miles/w), and no pain with a minimum of 15 single-leg active heel raises.

Local Pain Thresholds
The overall meta-analysis of local PPT documented an increased sensitivity in affected subjects (SMD: −1.54; 95% C.I. −1.92, −1.16; p < 0.001) (Figure 2). A decreased local PPT was found in 22 out 23 studies. Only Plinsinga et al. found a similar local PPT between healthy controls and patients with Achilles tendinopathy [14]. The sub-analysis, based on specific tendinopathy, documented a significant difference in local PPT only in patients suffering from epicondylalgia (SMD: −1.59; 95% C.I. −2.06, −1.12; p < 0.001) and greater trochanter pain syndrome (SMD: −1.49; 95% C.I. −1.94, −1.03; p < 0.001). No significant difference was documented in the sub-analyses on shoulder impingement syndrome, patellar tendinopathy, and plantar heel pain even though all single studies found an increased sensitivity in the affected patients. Similarly, no significant difference was found in the sub-analysis on Achilles tendinopathy, with three studies documenting an increased sensitivity in the affected patients and one study documenting a similar PPT between healthy and affected subjects. The overall analysis on HPTs found no difference between healthy and affected subjects with only the study of Coombes et al. documenting an increased sensitivity in patients with epicondylalgia and the study of Eckenrode et al. documenting an increased sensitivity in patients with Achilles tendinopathy [22,23]. No significant difference was documented in all sub-analyses based on the specific tendinopathy (Figure 3). A signifi- The overall analysis on HPTs found no difference between healthy and affected subjects with only the study of Coombes et al. documenting an increased sensitivity in patients with epicondylalgia and the study of Eckenrode et al. documenting an increased sensitivity in patients with Achilles tendinopathy [22,23]. No significant difference was documented in all sub-analyses based on the specific tendinopathy (Figure 3). A significant difference (SMD: 0.47; 95% C.I. 0.21, 0.72; p < 0.001), indicating an increased sensitivity in affected subjects, was found in the meta-analysis evaluating CPTs with the studies of Coombes et al. and of Ruiz-Ruiz et al., both documenting an increased sensitivity to cold stimuli in subjects with epicondylalgia, and the study of Plinsinga et al. documenting an increased sensitivity in the subjects affected by greater trochanter pain syndrome [22,38,41]. The sub-analysis on lateral epicondylalgia and on greater trochanter pain syndrome found a significant difference between groups (SMD: 0.68; 95% C.I. 0.46, 0.90; p < 0.001; SMD: 0.48; 95% C.I. 0.07, 0.89; p < 0.001) (Figure 3).
The overall analysis on distant PPTs reported a significantly increased sensitivity in patients with tendinopathies (SMD: −1.01; 95% C.I. −1.67, −0.38; p = 0.009) ( Figure 5). In particular, 11 studies reported lower distant PPT and 8 studies documented no difference between groups. Only the sub-analysis on greater trochanteric pain syndrome found a significantly increased sensitivity in the affected patients (SMD: −0.58; 95% C.I. −0.99, −0.17; p = 0.01). None of the other sub-analyses based on a specific tendinopathy documented an increased pain sensitivity in the affected patients. In particular, an increased sensitivity was documented in two out of three studies on shoulder impingement syndrome, in five out of six studies on epicondylalgia, in one out of two of the studies on patellar tendinopathy, in one out of five studies on Achilles tendinopathy, and in one out of two studies on plantar heel pain.

Patient Characteristics Influencing Pain Sensitivity
The meta-regression showed a significant influence of patient activity level on local PPT, with an increased difference in pain threshold in studies on athletes (β = −1.1, p = 0.02, R 2 19.2%). A significant influence on the results was also found for patient age with less difference in local PPT between healthy and affected subjects in studies including older patients (β = 0.04, p = 0.05, R 2 12.9%). No influence on the results of the analysis on local PPT was documented for the duration of symptoms.
When the meta-regression was conducted on the results of the meta-analyses of contralateral and distant PPTs, no significant influence of patient age, activity level, and duration of symptoms was found. Meta-regressions based on results of the meta-analyses of HPT and CPT, were not possible due to the low number of studies reporting these outcomes.   nopathy documented significantly lower PPTs in patient groups (SMD: −0.88; 95% C.I. −1.25, −0.51; p < 0.001; SMD: −1.01; 95% C.I. −1.51, −0.52; p < 0.001; SMD: −1.93; 95% C.I. −2.55; −1.32; p < 0.001). No significant difference in contralateral PPTs was documented in the sub-analyses on shoulder impingement syndrome and Achilles tendinopathy, with all studies on these tendinopathies reporting no differences between groups except for the study of Vallance et al. on Achilles tendinopathy [44]. The overall analysis on distant PPTs reported a significantly increased sensitivity in patients with tendinopathies (SMD: −1.01; 95% C.I. −1.67, −0.38; p = 0.009) ( Figure 5). In particular, 11 studies reported lower distant PPT and 8 studies documented no difference between groups. Only the sub-analysis on greater trochanteric pain syndrome found a significantly increased sensitivity in the affected patients (SMD: −0.58; 95% C.I. −0.99, −0.17; p = 0.01). None of the other sub-analyses based on a specific tendinopathy documented an increased pain sensitivity in the affected patients. In particular, an increased sensitivity was documented in two out of three studies on shoulder impingement

Patient Characteristics Influencing Pain Sensitivity
The meta-regression showed a significant influence of patient activity level on local PPT, with an increased difference in pain threshold in studies on athletes (β = −1.1, p = 0.02, R 2 19.2%). A significant influence on the results was also found for patient age with less difference in local PPT between healthy and affected subjects in studies including older patients (β = 0.04, p = 0.05, R 2 12.9%). No influence on the results of the analysis on local PPT was documented for the duration of symptoms.
When the meta-regression was conducted on the results of the meta-analyses of contralateral and distant PPTs, no significant influence of patient age, activity level, and duration of symptoms was found. Meta-regressions based on results of the meta-analyses of HPT and CPT, were not possible due to the low number of studies reporting these outcomes.  [25,29,30,38], whereas Van Wilgen et al. [48] and Vallance et al. [44] included only males. The selection and the definition of controls were rated appropriate in all studies including a control group. Control groups were stratified by age and other variables in all studies except for those of Paul et al. [36] (which included older subjects and predominantly Caucasian subjects in the patients group), Plinsinga et al. [40] (which included subjects with a higher BMI in the patients group), Plinsinga et al. [38] (which included subjects with a higher BMI in the patients group), and Riel et al. [14] (which included more females and patients with a higher BMI in the patients group). The ascertainment of exposure was appropriate in all studies, and the same method was used for both groups.

Discussion
The main finding of this study is that pain sensitisation is a feature of tendinopathies, as attested by the meta-analyses on local, contralateral, and distant PPTs. However, while results indicating a lower local PPT in the affected patients were consistent among different studies, results regarding contralateral and distant pain thresholds were conflicting among different studies and tendon diseases.
PPTs, defined as the minimum level of pressure at which patients report pain when an increasing pressure is applied at a specific site with the use of an algometer, are a reliable method to assess pain sensitivity in musculoskeletal pain diseases [49]. These tests may be performed at the original site of pain or at distant uninvolved sites. The presence of a decreased pain threshold at the site of the disease (also referred as hyperalgesia) seems to be due to the sensitisation of the peripheral nociceptors (C-fibres) of deep somatic tissues [50], and it is usually referred to as peripheral sensitisation [51]. Conversely, central sensitisation implies the amplification of the sensory input with an increase in the excitability of the neurons of the dorsal horn, leading to an increased responsiveness of the central nervous system to nociceptive stimuli [51]. This process, after an initial phase characterized by the expansion of the area of pain with hyperalgesia and allodynia, leads to the spread of hypersensitivity also to distant and non-injured areas (secondary hyperalgesia) [52]. As a consequence, the evaluation of PPTs at distal and contralateral sites may be considered a diagnostic tool able to detect the involvement of the central nervous system in the enhancement of pain perception [53].
In this light, the results of the present meta-analysis document the presence of pain sensitisation in chronic tendinopathies, underlying, in particular, the contribution of peripheral pain sensitisation. In fact, while results on local PPTs clearly and consistently demonstrate an increased sensitivity at the affected site, the presence of widespread hyperalgesia was less commonly reported. Indeed, even though the overall results of the meta-analyses show the presence of an augmented contralateral and distant sensitivity, only a part of the studies was able to detect lower PPTs in the affected subjects. Notably, the results of the evaluation of contralateral and distant PPTs differed among various tendinopathies. In particular, for lateral epicondylalgia, a widespread hypersensitivity was reported in six out of seven studies and in four out of five studies evaluating contralateral and distant pain thresholds, respectively. The only study on GTPS documented a higher distant PPT in affected subjects. Results on shoulder impingement syndrome, patellar tendinopathy, Achilles tendinopathy, and plantar heel pain were more conflicting with widespread sensitivity reported in two out of five studies, one out two studies, two out of six studies, and one out of two studies, respectively.
These findings support the idea that tendinopathies may differ in terms of underlying pain processing alterations. A recent review by Rio et al. distinguished between upper limb and lower limb tendinopathies suggesting there was more consistent evidence for widespread analgesia in upper limb tendinopathies [54]. However, their results on upper limb tendinopathies were led by the results on lateral epicondylalgia (in which the influence of pain sensitization on patient pain perception is better studied), whereas the heterogenous results on lower limb tendinopathies were influenced by the different diseases considered and by the lower number of studies included. Simply distinguishing between upper and lower limb tendinopathies may thus lead to an oversimplification, merging the results of various diseases with a different etiopathogenesis that potentially influences the complex mechanism of pain sensitization [55]. Moreover, as well as possible etiopathological differences among different tendinopathies, patient characteristics might also play an important role. Mc Auliffe et al. [15] and Rio et al. [54] hypothesized that patients included in studies on upper limb tendinopathies are usually older and less active, which might influence the results of QST protocols, as a younger age and a higher activity level have been suggested to present a lower risk of developing central sensitisation [56,57]. However, while confirming the overall influence of age and activity level, this meta-regression analysis did not support the hypothesis of Mc Auliffe et al. Both age and activity level were found to correlate with differences in PPTs, but a higher difference was reported in studies including younger patients or athletes. Conversely, the meta-regression reported no influence on the results of the meta-analysis of symptoms' duration, a factor that is often considered as important for the development of pain sensitization [58]. These controversial findings suggest a complex interplay of different factors, as the results of studies on various tendinopathies could not simply be explained by these variables. To this regard, the more common and broader abnormalities in pain processing in patients suffering from lateral epicondylalgia may be due to the characteristics of the disease itself rather than to the characteristics of the affected patients.
Among different tendinopathies, studies on epicondylalgia showed more evidence of disease features that directly support the influence of the central nervous system on perceived pain. Burns and colleagues, using surface electromyography and transcranial magnetic stimulation, demonstrated less GABA A -and GABA B -mediated intra-cortical inhibition, and less intra-cortical facilitation in the motor cortex in individuals with lateral epicondylalgia compared with healthy controls [59]. The expression of mediators involved in the development of neurogenic pain, such as glutamate [60], substance P [61], and neurokinin 1 [62], has been extensively documented in patients affected by lateral epicondylalgia, while inflammatory cells do not seems to be increased [61]. Conversely, inflammatory cells seem to play a more important role in other tendinopathies such as rotator cuff tears, patellar tendinopathy, and Achilles tendinopathy [63]. Moreover, Debenham et al. [64], although showing abnormal local two-point discrimination thresholds in subjects with Achilles tendinopathies, demonstrated similar values between the contralateral side of affected patients and unaffected site of healthy subjects, suggesting that sensitive alterations involve only the affected side in Achilles tendinopathy. Overall, the current literature findings support a higher involvement of the central nervous system in lateral epicondylalgia compared to other tendinopathies, where other mechanisms, such as local inflammatory processes, may play a stronger role in generating pain [40,65].
As well as PPT, the most complete QST protocols include the evaluation of thermal pain thresholds [66]. The data of CPTs, although included in a lower number of studies, confirmed the results of local PPT evaluations: an increased sensitivity was detected in the overall analysis. However, when the single tendinopathies were considered, only studies on lateral epicondylalgia were able to detect an increased sensitivity in the affected subjects. Regarding HPT, no difference was documented between healthy and affected subjects in the overall analysis and in all different tendinopathies. These results may be due to the fact that, whereas the mechanical stimuli is considered the best method to detect sensitisation of the deep somatic tissues' (i.e., joint and muscle) nociceptors, thermal pain threshold may be less suitable, detecting, at best, the sensitisation of cutaneous nociceptors [51]. Moreover, a higher variability was documented in the literature for thermal pain thresholds compared to PPTs [67].
The literature presents limitations that are reflected in the present meta-analysis. The number of included studies in some of the sub-analyses is low, thus limiting the strength of these findings. Some planned analyses, such as those on distant and contralateral thermal pain threshold, temporal summation, and conditioned pain modulation, could not be performed due to lack of data. Moreover, there was a high heterogeneity in the characteristics of patients from the different trials, with a wide range of mean age (23.3 to 65.7 years) and mean pain duration (3 months to 10.6 years). A meta-regression analysis to evaluate the influence of patient-related factors was not always possible. Finally, to provide an estimate of the prevalence of pain sensitisation in chronic tendinopathies, the systematic review included also studies using questionnaires such as painDETECT, the Central Sensitization Index, and Dolour Neuropathic 4 that were initially developed to evaluate neuropathic pain and rely on questions that may reflect a broader definition of sensitivity, which includes depression, anxiety, stress, and neuroticism. However, despite the abovementioned limitations, especially pertaining the possibility to perform the planned sub-analyses, the primary analysis of the available literature offered clear results with important insights. In fact, compared to the previous literature analyses on this topic, this meta-analysis took advantage of several recent publications on pain sensitisation in tendinopathies, which allowed a quantitative synthesis on the comparison of healthy and affected subjects in terms of pain thresholds to be provided. An overall higher sensitivity was detected in patients with tendinopathies, with more evidence of pain sensitisation in patients affected by lateral epicondylalgia. Future studies should focus on the identification of pain mechanisms contributing to symptoms in different tendon diseases, in order to develop more effective treatment approaches for patients affected by tendinopathies.

Conclusions
Tendinopathies are characterized by pain sensitisation but, while features of both central and peripheral sensitisation can constantly be detected in lateral epicondylalgia, results on other tendinopathies were more conflicting, being conclusive only on the presence of peripheral sensitisation. In addition to pathophysiological differences among tendinopathies, patient characteristics are possible confounders that should be taken into account when addressing pain sensitisation.